The present invention pertains to an optical absorption measuring instrument for determining the percentage of a component in a fluid.
Optical absorption measuring instruments, e.g., in the form of infrared optical measuring instruments, measure the absorption of infrared beams at a wavelength specific of the component to be detected. The amount of the absorption is an indicator of the percentage of the component in the fluid. There are various designs of such measuring instruments with an infrared radiation source or with a plurality of infrared radiation sources, with the measuring beam passing through an open or closed measuring chamber once or several times. The measuring beam reaches a detector or a plurality of detectors, which are sensitive to the corresponding infrared wavelengths. If a plurality of detectors are used, one of which is a measuring detector and the other a reference detector, the measuring beam is distributed possibly uniformly among the detectors. So-called beam splitters are used to split the measuring beam.
An infrared optical measuring instrument of this type has been known from DE 37 36 673 C2. The measuring beam emitted by an infrared radiation source passes through a measuring chamber, in which the gas component, whose concentration is to be monitored, is located. A ray beam emitted by a radiation source is reflected back by means of two spherical mirrors, which are arranged opposite the radiation source, and is imaged as a focused luminous beam on a detector, which is located at the said limiting surface of the measuring chamber as the radiation source. The individual luminous beams now reach the detector at different reflection angles. The drawback of the prior-art device is that the optical imaging on the receiving surface of the detector is changed by a change in the geometry of the radiation source or due to changes in the position of the mirrors due to changes in temperature. Drift and temperature effects are thus superimposed to the measured absorption signal.
The basic object of the present invention is to improve a measuring instrument of the above-described type such that changes in the measuring radiation due to drift and temperature have only a limited effect on the measuring result.
According to the invention, an optical absorption measuring instrument is provided for determining the percentage of a component in a fluid. A radiation source emitting measuring beams and at least one detector with a receiving surface sensitive to the measuring beams is at a first limiting surface of a measuring chamber containing the fluid. At least two plane mirrors are at a second limiting surface of the measuring chamber. The second limiting surface is located opposite the first limiting surface. The plane mirrors are positioned at the second limiting surface such that the measuring beams emitted by the radiation source are deflected to the detector. The surface of each plane mirror is dimensioned to be such that its illuminated surface in the area of the detector is larger than the receiving surface.
The advantage of the present invention is essentially that by using at least two plane mirrors for deflecting the beam, the radiation source is not imaged on the receiving surface, but the receiving surface of the detector is illuminated two-dimensionally, and the surface of each plane mirror is dimensioned to be such that the illuminated surface in the area of the detector is larger than the receiving surface of the detector. Due to the overlap of the illuminated surface and the receiving surface, it is achieved that the full receiving surface is illuminated even in case of drift and temperature effects and no variations can be observed as a result in the intensity of the measuring radiation in the area of the detector in case of temperature-related changes in the position of the plane mirrors. The overlap between the illuminated surface and the receiving surface is dimensioned to be such that the measuring beam still reaches the full receiving surface even in case of the most unfavorable constellation of the parameters. The tolerance range for the overlap can be determined most simply experimentally by exposing the measuring chamber to different temperatures and checking at each temperature whether the receiving surface of the detector is fully illuminated.
The present invention is not limited to the pure gas analysis, but it is also suitable generally for the determination of the percentage of a component, gas or liquid, in a gas or liquid sample.
It is especially advantageous to arrange a reference detector at the first limiting surface of the measuring chamber in addition to the detector. The reference detector has a reference receiving surface, which is illuminated by one of the plane mirrors, while the other plane mirror deflects the measuring beam to the detector. The surface of the plane mirror directed toward the reference detector is dimensioned to be such that the illuminated surface in the area of the reference detector is larger than the reference receiving surface.
The overlap between the illuminated surface and the receiving surface in the area of the detector or the reference detector is advantageously greater than 10% relative to the respective receiving surface. Experiments revealed that an overlap of about 10% to 20% is sufficient to suppress drift and temperature effects.
It is especially useful to design the limiting contour of the plane mirrors such that it is adapted to the receiving surface of the detector or of the reference detector. Thus, a rectangular mirror shape is suitable for a rectangular receiving surface, a round plane mirror in case of a round receiving surface, and a hexagonal mirror shape, which can be advantageously arranged in a honeycomb-like manner at the second limiting surface of the measuring chamber, in the case of a hexagonal receiving surface.
It is especially advantageous to design the plane mirrors at the second limiting surface in the form of a plane mirror matrix, wherein a first group of plane mirrors is directed toward the detector and a second group of plane mirrors toward the reference detector.
Due to the use of a plurality of plane mirrors in the form of a plane mirror matrix, contamination effects can be compensated in an especially simple manner and, moreover, no beam splitter is necessary, which would split the measuring beam between the detector and the reference detector behind the measuring chamber, because the individual plane mirrors can be positioned in the case of the plane mirror matrix such that the measuring beams are reflected directly to the detector and the reference detector.
The infrared optical measuring instrument can be manufactured in an especially simple manner and inexpensively by means of a plane mirror matrix, because the plane mirror matrix can be manufactured in connection with the second limiting surface as a one-piece molding, e.g., in the form of a plastic injection-molded part.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.